The potential use of controlled fusion for the commercial production of energy and/or energy products is a field in which a vast amount of research and development work, involving the expenditure of huge sums of money, is presently underway. A number of plasma confinement schemes are under study, of which toroidal confinement systems are proposed, and have been tested, and tokamaks are examples of toroidal confinement systems.
The tokamak fusion concept confines a plasma by the application of a toroidal magnetic field created by the toroidal magnet and a poloidal magnetic field created by the introduction of a large plasma current around the torus. This plasma current is basically produced by an induced electromotive force, emf, provided by poloidal field coil turns of which a central solenoidal ohmic heating field coil is the dominant coil. The effect is basically that of a transformer with the ohmic field coil operating as the primary and the plasma operating as a single turn secondary. In addition to contributing to plasma confinement, the poloidal field coil set also provides plasma heating through resistive absorption in the plasma as well as stability against horizontal and vertical displacements of the plasma. Tokamak plasma currents also may be provided by the application of radiofrequency power to the plasma, an effect that contributes to confinement, heating and, perhaps, steady state tokamak operation.
Tokamaks, as is true of any of the proposed fusion plant reactor embodiments, must not only be able to achieve and maintain acceptable plasma confinement conditions to enable the controlled fusion reaction, but also must meet criteria for commercial acceptability. These criteria for commercial acceptability involve many highly interrelated factors, including not only the power density available within the plasma when the machine is operating, but also such factors as recirculating power fraction (i.e, the amount of power necessary to drive the system to ignition and to maintain the necessary plasma confinement for burn), and the operating performance of the fusion reactor.